The invention is directed to air drying apparatus and methods, including air drying apparatus designed for use in transit vehicles, locomotives, trucks and other similar vehicles or mechanical apparatus employing compressed air systems.
Trucks, buses, locomotives, transit vehicles and the like typically contain compressed air systems that operate brakes, automatic doors, horns, and the like. Air that emerges from air compressors in such systems often contains moisture and lubricating oil mist vaporized within the air. Such moisture and oil mist contaminants must be continuously removed from the compressed gas handling system to prevent failure of the system due to build-up of water or sludge in compressed air lines and air operated equipment.
Air dryers may be used to reduce water vapor content of the air which will further reduce the likelihood of liquid water condensing in the air lines and equipment downstream of the air dryer. One problem with such air dryers is that particulate material or soot may clog the filters of such dryers, which sometimes causes the filters to malfunction. When a filter breaks, unfiltered contaminated gas undesirably may seep through the break, preventing the dryer from operating effectively.
Dessicant dryers commonly used to process onboard compressed air on locomotives often have proved inadequate. Dessicant towers usually require a relatively large canister of dessicant material to perform well. In locomotive applications, the peak compressed air demand of a train often exceeds the air processing capability of the most commonly available tower-type dessicant dryers.
Membrane-type dryers are known for reducing the relative humidity of compressed air streams, as shown in U.S. Pat. No. 6,126,724 to Martin et al (the xe2x80x9cMartin patentxe2x80x9d). The apparatus disclosed in the Martin patent employs membranes that selectively block nitrogen and oxygen passage, but allow gaseous water (i.e. water vapor) molecules to pass through and be discarded. In general, the efficiency of water vapor removal using membrane drying systems is dramatically improved when higher flow pressures are used. The mass transfer driving force is the difference in partial pressures of water vapor on each side of the membrane.
Membrane dryers are susceptible to fouling if excess amounts of oils, soot, liquid water, or other contaminants become lodged in the relatively sensitive membrane material. One problem in using membranes for drying applications is that contaminants may become undesirably lodged in the membrane, reducing or eliminating the ability of such membranes to perform gaseous separations.
Sweep air is required for the operation of such membrane drying systems. In general, the greater the difference in water vapor partial pressure across a membrane, the more efficient the transfer of undesirable water vapor out of the system. Sometimes air which has already been cleaned and dried is released downstream of the membrane through an orifice. This dried air may be used as xe2x80x9csweep airxe2x80x9d to flow back across or along the membrane. This sweep air may serve to provide a favorable partial pressure for efficient drying operations. Usually, this sweep air is at or close to atmospheric pressure.
Because sweep air usually approximates atmospheric pressure, the level of flow through such a downstream orifice may be undesirably dictated by the upstream pressure level. That is, at process pressures below about 30 psia, the flow through the orifice may vary as a function of pressure. Conversely, at pressures above about 30 psia, the flow may become constant regardless of the amount of process pressure applied. This sometimes results in a drying apparatus or system that, when operated at flows below nominal, uses greater amounts of sweep air than necessary. At flows above nominal, the amount of sweep air used may be a much smaller percentage of total process flow, resulting in less dew point depression accomplished by the air dryer. Thus, the amount of sweep air employed in systems using a downstream orifice may be undesirably dictated by the upstream process flow pressure.
In summary, it would be desirable to provide a system, apparatus and method for removing contaminants such as water vapor, oils, and the like from compressed air lines or sources effectively and efficiently. A method and apparatus for filtration that is capable of removing solid particulates, such as soot, and other contaminants such as entrained oil and water, without fouling or disabling membrane filters, would be highly desirable. Furthermore, a system that is capable of varying the amount of sweep air employed, as a function of the process flow and pressure, would be very desirable. And finally a method of treating the air with as high a process air pressure as possible would be highly desirable.
In the invention, a processing apparatus for removing contaminates from a process flow of air is provided. The apparatus may employ any number of membrane filters. One or more of such membrane filters include a filtration media made up of a number of individual tubules, within the membrane filter being adapted to receive a process flow of contaminated air along a process flow pathway. The filtration media assists in separating from the process flow gaseous water vapor to dry the air within the process flow.
Some applications of the invention employ one or more pre-filtering stages or steps in which aerosol mist, oils, soot, liquids or other particulates may be removed from the air stream prior to the air stream reaching the membrane filter. However, such pre-filtering apparatus or steps are optional, and may not be included at all, depending upon the application.
Furthermore, the apparatus includes a sweep chamber, the sweep chamber being positioned adjacent the filtration with the sweep air flowing adjacent to the individual fiber tubules.
Optionally, a flow metering device may be positioned within the process flow pathway, and downstream from the membrane filter. The flow metering device may be adapted to divert a portion of the process air flow as sweep air into the sweep chamber. In the apparatus, the sweep air may be in flowing fluid communication with the filtration media, adjacent to the membrane filter.
In other applications of the invention, it may be desirable to provide an optional minimum pressure valve downstream of the sweep chamber. Such a minimum pressure valve (or regulator) could be configured to maintain a minimum threshold pressure of process flow, increasing the efficiency of the apparatus.